Nickel base diffusion bonded article

ABSTRACT

A nickel base, boron including bonding powder for use in bonding alloy members is provided with improved toughness and ductility through the elimination of the boride former and precipitation strengthener titanium and the boride formers and solution strengtheners molybdenum and tungsten, which for some applications have been included in age hardeneable bonding powders. Further melting point reduction can be obtained through the addition of elements selected from tantalum and hafnium in the sum of 2 - 4 weight percent. An unique joint produced in an article between alloy members using such bonding powder is characterizied by a recrystallized, non-planar microstructure including predominantly discrete, dispersed borides rather than a continuous planar boride phase.

Unite States Patent [191 [111 3,820,961 Zelahy June 28, 1974 [541 NICKEL BASE DIFFUSIOYN BONDED ARTICLE [75] Inventor: John W. Zelahy, Cincinnati, Ohio [73] Assignee: General Electric Company,

Cincinnati, Ohio [22] Filed: Mar. 16, 1973 [21] Appl. No.: 341,882

Related US. Application Data [62] Division of Ser. No. 186,490, Oct. 4, 1971, Pat. No.

[52] US. Cl. 29/194, 75/171 [51] Int. Cl B23k 31/02 [58] Field of Search 75/15 AA, 15 BA, 171; 29/194 [56] References Cited UNITED STATES PATENTS 2,714,760 8/1955 Boam et al 29/498 X 3,303,024 2/1967 Cape 75/171 3,632,319 H1972 Hoppin et a1. 29/504 x 3,692,501 9/1972 Hoppin 29/l94 X 3,700,427 10/1972 Hoppin et al. 75/171 X Primary ExaminerL. Dewayne Rutledge Assistant ExaminerArthur J. Steiner Attorney, Agent, or Firm-Lee l-l. Sachs; Derek P. Lawrence [5 7] ABSTRACT A nickel base, boron including bonding powder for use in bonding alloy members is provided with improved toughness and ductility through the elimination of the boride former and precipitation strengthener, titanium and the boride formers and solution strengtheners molybdenum and tungsten, which for some applications have been included in age hardeneable bonding powders. Further melting point reduction can be obtained through the addition of elements selected from tantalum and hafnium in the sum of 2 4 weight percent. An unique joint produced in an article between alloy members using such bonding powder is characterizied by a recrystallized, non-planar microstructure including predominantly discrete, dispersed borides rather than a continuous planar boride phase.

3 Claims, No Drawings invention.

BACKGROUND OF THE INVENTION Recent advances in that technology relating to solid state pressure diffusion bonding include the development of vacuum hot press furnace apparatus capable of developing high vacuum as well as high temperature and pressure around components of an article to be joined by solid state diffusion bonding. The combination of such technology with that relating to brazing employing a bonding alloy led to an invention described in co-pending application Ser. No. 841,093, filed July 11, 1969, now U.S. Pat. No. 3,632,319, and assigned to the assignee of the present invention. Such invention, one form of which is sometimes referred to as activated diffusion bonding, provides an improved diffusion bonded joint between alloy members through the use of a bonding powder the composition ofwhich is matched with that of the alloy members being joined and which includes certain melting point depressants selected from B, Si, Mn, Cb, Ta and their mixtures. Only relatively slight aligning pressures are required between members to be joined to effect a sound bond under relatively high vacuum conditions.

It has been recognized through microprobe analysis, that certain activated diffusion bonded joints in nickel base alloys retain certain melting point depressants combined with Mo, W and Ti resulting in brittle, continuous phases and hence lower joint ductilities.

SUMMARY OF THE INVENTION It is a principal object of the present invention to provide a nickel base bonding powder of improved toughness and ductility through the elimination of those elements which tend to combine with melting point depressants in the bonding alloy to generate in the joint area phases which tend to embrittle the joint.

Another object is to provide such a bonding powder including melting point depressants which have a tendency to migrate into the parent'metal, in order to combine with such eliminated elements, leaving the joint region relatively free of brittle phases.

Still another object is to provide between bonded members a joint having an unique microstructure which provides the joint with improved fracture toughness and ductility.

These and other objects and advantages will be more clearly understood from the following detailed description and the examples all of which are meant to be typical of rather than limiting on the scope of the present invention.

Briefly, the present invention provides a nickel base bonding powder consisting essentially of a matrix of Ni.

2 lected from Cr and Co with the balance Ni and incidenal impurities, along sa -.2-it5 Bt2:% 4 14 to about 0.2% C, and up to about 4 percent of eTements selected from Ta and Hf. A preferred form of the bonding powder consists essentially of, by weight, 14 17% Q .ZZ QQL- Z2A1,...L5r24% .B 214% {Hf Ta), up to about 0.2% C, with the balance Ni and infififi l i .t mflwwam E a An improved joint provided in a diffusion bonded article through use of such bonding powder has improved toughness and ductility from a recrystallized, nonplanar microstructure which includes primarily discrete, dispersed borides in lieu of a continuous, planar boride phase frequently found in joints made with currently used bonding alloys.

DESCRIPTION OF THE PREFERREB EMBODIMENTS Metallographic and chemical examination of activated diffusion bonded joints of certain nickel base superalloys of the type precipitation strengthened primarily throughthe elements A] and Ti and solution strengthened primarily through the elements W and Mo, disclose the presence of embrittling phases. These are believed to be primarily as a result of the introduction of melting point depressants, such as Si or B or both in an age hardenable bonding powder including all of such precipitation and solution strengthening elements.

lt has been found, according to the present invention, that improved joint toughness and ductility can be provided in such activated diffusion bonded joints through the provision of a NiCr base bonding powder which does not include the elements Mo, W and Ti but includes B as a melting point depressant. The addition of other melting point depressants selected from Hf and Ta further improves melting characteristics as well as fragmenting and recrystallizing the joint area. More specifically, elimination of Mo and W appreciably improves impact resistance whereas elimination of Ti was beneficial from the standpoint of producing a nonplanar crack propagation resistant joint.

During detailed evaluation of the present invention, a variety of bonding alloys were prepared by argon arc melting repeatedly to provide homogeneity, after which they were crushed in a forge hammer. Flow panel tests were conducted in vacuum to determine the liquidus and solidus temperatures along with flow characteristics and cleanliness. Selected for use as a parentmetal in members to be joined in the evaluation of the present invention was a nickel base alloy, sometimes referred to as Rene alloy, and consisting nominally of, by weight, 0.17% C, 14% Cr, 5% Ti, 0.015% B, 3% wwwacwmmr. wi hth ance essentially nickel and incidental impurities.

Of particular interest in connection with the evaluation of the present invention are the alloys the compositions for which are'shown in Tablel and prepared as qssqribs ausa--.

TABLE 1 Alloy These specimens were made up of pairs of "/s inch diameter X 1 inch long bars positioned end to end with the bonding alloy placed in the joint as a 0.010 inch thick, 50 percent dense preform of bonding alloy and acrylic binder. The bars were then arranged vertically in groups of three and a dead weight load was applied. The specimens were then brazed under the various conditions described in connection with the specific examples in the following Table ll. Impact testing was conducted at room temperature on un-notched, 0.300 inch diameter X 2 inches long bars using a. pendulum impact tester. The specimens were supported at the ends with theiimpacter striking at the bonded joint.

TABLE ll lmpact Data at Room Temp.

Joint Bonding (alhall' of specimens tested failed in parent metal (h nll nfspccimuns tcslvd failed in part-m metal The data of Table ll shows that the'joint bonded with the bonding powder of the present invention, which eliminates the presence of Mo. W and Ti, results in a joint having an average impact strength which not only can equal but can exceed the parent metal impact strength. As shown by that data, the specimens bonded at temperatures of about 2200F or below, for times in excess of /2 hour, are specifically preferred in that they equal and generally exceed the parent metal impact strength. ltshould be noted that failure of specimens bonded below about 2200F resulted in every case in failure in the parent metal rather than in the joint area. Thus, with the proper selection of processing conditions,,the bonding powder of the present invention can provide tough and ductile joints between superalloy members.

Joint microstructures correlated well with the impact results. Joints having high impact strength exhibited a recrystallized zone with dispersed borides whereas the weaker joints exhibited larger, more continuous borides. Joints made at the higher pressures and for longer times, produced joints with less porosity an fewer borides.

The elimination of the boride formers Mo, W and Ti from the bonding powder results in a joint average impact strength of more than three times that produced using known materials. However, through the inclusion of elements selected from Ta and Hf, still further and significantly greater improvement in joint impact strength can be obtained to exceed the impact strength of the parent metal surrounding the joint. Thus, a preferred form of the present invention not only eliminates Mo, W and Ti from the bonding'powder composition, but specifically includes elements selected from Hf and Ta.

Of the elements included in the improved bonding alloy of the present invention, it has been recognized that amounts of B and of the elements of the group Ta and l-lf are more critical. In respect to B, less than about 1.5 percent results in too high a liquidus temperature and poor flow characteristics. Greater than about 2.4 percent B results in the formation of excess borides and too much liquid. In respect to the elements Ta and l-lf in the specifically preferred form of the present invention, less than about 2 percent does not help recrystallization of the joint and is not significantly effective in reducing the liquidus temperature; greater than about 4 percent shows no additional benefit to the alloy and adds to its cost.

In respect to C, the inclusion of up to about 0.2 weight percent appears to have no appreciable effect on the alloy. However, greater than about 0.2 percent tends to raise the melting temperature.

Of the other elements included in the bonding powder of the present invention, the more significant is Cr in its preferred upper range of about 17 percent. Above that amount, there may in some alloys be a tendency to form the embrittling sigma phase which is undesirable.

As will be recognized by those skilled in the art of metal joining, there are various modifications which can be made within the scope of the present invention, some examples of which have been specifically presented.

What is claimed is: 1. An article including an improved diffusion bond ed joint portion between a plurality of nickel base superalloy members, the joint portion:

consisting essentially of, by weight, 1.5-2.4% B, 2- %A1 ap.t9 ab u 4 per en qts ement s lected from the group consisting of Ta and Hf the sum of such elements when selected being 2 4 percent, up to about 0.2% C, with the balance Cr, Co, Ni and incidental impurities; and

characterized by a recrystallized, non-planar microstructure including predominantly discrete, dispersed borides.

2. The article of claim 1 in which the joint portion consists essentially of, by weight:

21 Ali... I

2 4 percent of elements selected from the group consisting of Ta and Hf;

Cr and Co in the sum of 8 30 percent;

up to about 0.2% C;

with the balance Ni and incidental impurities.

3. The article of claim 2 in which the Cr is 14 17 percent and the Co is 8 12 percent. 

2. The article of claim 1 in which the joint portion consists essentially of, by weight: 1.5 - 2.4% B; 2 - 5% A1; 2 - 4 percent of elements selected from the group consisting of Ta and Hf; Cr and Co in the sum of 8 - 30 percent; up to about 0.2% C; with the balance Ni and incidental impurities.
 3. The article of claim 2 in which the Cr is 14 - 17 percent and the Co is 8 - 12 percent. 